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Iridium-Catalyzed Continuous Hydrogen Generation from Formic Acid and its Subsequent Utilization in a Fuel Cell: Towards a Carbon Neutral Chemical Energy Storage Miklos Czaun, Jotheeswari Kothandaraman, Alain Goeppert, Bo Yang, Samuel Greenberg, Robert B. May, George A Olah, and G. K. Surya Prakash ACS Catal., Just Accepted Manuscript • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on September 28, 2016

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Iridium-Catalyzed Continuous Hydrogen Generation from Formic Acid and its Subsequent Utilization in a Fuel Cell: Towards a Carbon Neutral Chemical Energy Storage Miklos Czaun*, Jotheeswari Kothandaraman, Alain Goeppert, Bo Yang, Samuel Greenberg, Robert B. May, George A. Olah*, and G. K. Surya Prakash* Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, University Park Campus, Los Angeles, CA 90089 (USA), Fax: (+) 1-213-740-6679 ABSTRACT This study represents a notable step towards a potentially carbon neutral energy storage solution based on formic acid as a hydrogen/energy carrier. A catalytic system derived from IrCl3 and 1,3-bis(2'-pyridyl-imino)-isoindoline (IndH) in the presence of aqueous sodium formate showed high selectivity and robustness for hydrogen generation from formic acid (FA) at 90-100 °C under both high and moderate pressure conditions suppressing the formation of CO impurity. Being a solid substance, the catalyst can be recovered by a simple filtration, if necessary. Furthermore, addition of neat formic acid is sufficient to reuse the catalyst and maintain a constant flow of H2 and CO2 mixture and the stable performance of a coupled fuel cell. The easy to recycle catalyst did not show any loss of activity after 20 days of continuous use and similar activity was observed even a year after the original preparation. The reactor for formic acid decomposition provided a one to one ratio of H2/CO2 mixture that was coupled to a hydrogen/air proton exchange membrane (PEM) fuel cell to demonstrate a stable and continuous conversion of chemical energy to electricity. This integrated system embodies the first example of an indirect formic acid fuel cell, which can function, without the requirement of applying inert conditions and feed gas purification, for extended periods of time.

KEYWORDS: energy storage, hydrogen generation, formic acid decomposition, iridium chloride, indirect formic acid fuel cell. Introduction According to a report from the United Nations, the world population reached 7.3 billion as of the middle of 2015 indicating a growth of one billion in the last twelve years.

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Medium-variant projections estimate the world population to further increase by more than one billion people within the next 15 years and reach 11.2 billion by the end of the 21st century.1 The increase in the world’s population has been accompanied by an impressive technological development since the industrial revolution and has resulted in an ever increasing energy demand by humankind. In 2012, the total energy consumption of the world was approximately 104,400 TWh. The three most important fossil fuels, oil, natural gas and coal contributed 81% to the total energy production and 67 % of electricity production, according to the report of the International Energy Agency.2 The annual global carbon dioxide emissions from fossil fuel combustion and from industrial processes (mainly cement and metal production) reached a new record of 35.3 billion tonnes in 2013.3 In fact, carbon dioxide produced by the economic sector contributed to 84% of the total greenhouse gas emission in 2011.4 Nevertheless, a promising paradigm shift can be seen on the usefulness and utilization of carbon dioxide as a raw material.5-8 Environmental concerns, such as global warming and climate change together with technological developments, are paving the way for carbon dioxide to be seen not only as a harmful greenhouse gas but more like a useful industrial C1 feedstock.9,10 The needed CO2 can be captured from concentrated sources (e.g. flue gas) and eventually from ambient air as well.11-16 The incredibly large anthropogenic CO2 emission suggests that meeting our increasing energy need in a sustainable way will be exceedingly challenging. Nevertheless, the Sun may provide a sufficient amount of renewable energy for the generations to come, since the Earth receives more energy from sunlight striking its surface in one hour (4.3 × 1020 J) than all the energy consumed by humankind in a year (4.1 × 1020 J).17 The majority of renewable energy sources such as solar, wind, hydro and geothermal are used to generate electricity. Unfortunately, the most promising and scalable ones, solar and wind are intermittent and highly fluctuating in nature, making storage of obtained electrical power necessary. Storage of large amounts of electrical power is, however, a major challenge of our century due in part to some obstacles related to large scale conventional battery storage, e.g. using toxic or scarce metals, large battery size, limited number of charge/discharge cycles. Numerous approaches such as mechanical-, thermal-, electrochemical- and chemical energy storage were developed and tested in the last decades. Among the variety of substances that can be used for chemical energy storage, molecular hydrogen is the


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simplest compound, consisting of two hydrogen atoms only. Generating hydrogen by water electrolysis using electricity from renewable sources18 followed by storage- and finally conversion of hydrogen to electricity, on demand, would be a “clean” approach to store energy, since water, an environmentally benign material, is the only product of the hydrogen combustion.19 More than 60 million tonnes of hydrogen is utilized worldwide, mainly for ammonia, fertilizer and methanol production as well as in oil refineries. While 95% of hydrogen20 is produced by reforming fossil fuels such as steam reforming or partial oxidation of methane and coal gasification,21 a significantly smaller amount of hydrogen is obtained by other processes, e.g. biomass gasification or water electrolysis. Recently, as the amount of electricity generated from renewable energies has been increasing, more hydrogen is being produced by water splitting technologies.20 Although only 1% of the total H2 produced is utilized as an energy carrier (mostly for space applications), hydrogen in many aspects can be considered as an advantageous fuel.22,23 It is clean burning, gives only water when combusted and has a mass energy density three times as high as that of gasoline. However even though enormous progress has been achieved in hydrogen storage technologies,24 hydrogen is not yet a practical fuel, due to several technical difficulties associated with its storage and distribution.6 In addition to hydrogen, a variety of materials, such as, ammonia-borane,25 alkali borohydrides,26 hydrous hydrazine,26 methane, methanol,6,27 etc., have been considered for chemical energy storage. The practical applicability and feasibility of some of these materials, however, may suffer from their relatively high price, regenerability, toxicity and/or safety issues. Formic acid (FA) has been studied as a promising candidate for fuel cell applications because of its facile oxidation kinetics, high theoretical cell potential and low fuel crossover.28 In addition, formic acid can be prepared by synthesis from biomass,29-32 recycling carbon dioxide by catalytic hydrogenation33-35 or electrochemical reduction.3638

Biobased FA is a safe reagent for energy storage and chemical synthesis and its

production is economically appealing.39 Some sceptics argued on the feasibility of FA as a fuel due to its “toxic” nature. On the one hand, a simple comparison of LD50 values would suggest that FA is indeed more toxic than gasoline assuming oral dosing. On the other hand “consumers”, who buy any kind of fuel, regardless of its nature and toxicity, are not supposed to drink the fuel. In fact, FA is on the list of US Food and Drug Administration, among synthetic flavoring


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substances and adjuvants that may be added to food products40 and also among constituents that may be added to paper and paperboard used for food packaging.41 Not surprisingly, conventional liquid fuels such as gasoline and diesel fuel are not on these lists. Conversion of FA to electricity was shown in direct formic acid fuel cells (DFAFCs,42 Figure 1a) but these devices still need to be improved in terms of efficiency and durability. The energy stored in FA in the form of chemical bonds can also be converted to electricity in a way where the fuel is not fed to the fuel cell directly.43-46 FA, for example, can be decomposed to hydrogen and carbon dioxide47,48 (Figure 1c, (ii)). The hydrogen content of the gaseous product can be fed into a hydrogen/air proton exchange membrane (PEM) fuel cell, which is a well-known and established technology giving good efficiencies.49 The FA decomposition unit in combination with the hydrogen/air fuel cell constitutes an indirect formic acid fuel cell, shown in Figure 1b. Carbon dioxide that forms in the decomposition reaction can be recycled and converted back to FA, establishing a carbon neutral energy storage system (Figure 2).

Figure 1. Direct (a) and indirect (b) FA fuel cells and decomposition pathways for FA (c). (i) Decarbonylation or dehydration, (ii) dehydrogenation or decarboxylation.

As indicated in Figure 1c, the decomposition reaction of FA can also result in the formation of carbon monoxide and water. This decarbonylation pathway (i) should be suppressed as much as possible if one intends to utilize the gas mixture obtained by FA decomposition in a hydrogen/air PEM fuel cell, because of the deteriorative effect of carbon monoxide on the fuel cell Pt electrocatalyst. In fact, in addition to high activity


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and robustness, minimizing CO evolution in the course of FA decomposition is a major challenge in the catalyst development for hydrogen storage using HCO2H to H2 + CO2 or HCO2Na + H2O to H2 + NaHCO3 systems.50-53 Sabatier and Mailhe54 first reported the catalytic decomposition of FA using heterogeneous catalysts. Later, numerous iridium,55-61 ruthenium,45,47,48,62-65 iron,66-68 and rhodium69 derivatives were studied as homogenous catalysts for the decomposition of FA to hydrogen and carbon dioxide. Himeda et al. reported a particularly impressive TOF of 228,000 h-1 at 90 °C for the dehydrogenation of FA applying a [(Ir(Cp*)(Cl))2(thbpym)]2+ catalyst (Cp* = pentamethylcyclopentadienide, thbpym = 4,4’,6,6’-tetrahydroxy-2,2’-bipyrimidine).59 An even higher TOF of 322 000 h−1 at 100 °C was obtained by using a [Cp*Ir(2,4-(HO)2-pm-imidazoline)(OH2)]SO4 complex, which contains 6-(4,5-dihydro-1H-imidazol-2-yl)- pyrimidine-2,4-diol as a ligand.61 A variety of heterogeneous catalysts including SiO2, Al2O3, TiO2, MgO, ZnO, Fe3O4, Cr2O3, ThO2, etc. were reported for the dehydrogenation of FA but these catalysts required elevated temperatures (e.g. ≈280-330 ºC) for achieving high conversions.70,71 A smaller group of heterogeneous catalyst operates under more moderate conditions (e.g. room temperature). For example, a Ag-Pd core shell nanocatalyst72 produced 3.67 L H2 h-1 g-1 at 20 ºC with no detectable CO in the gas mixture. However, at 70 ºC, the selectivity suffered to a certain degree, producing a gas mixture containing 70 ppm CO. Review articles by Enthaler et al.73 and Laurenczy et al.51 summarizes the recent development in the decomposition of FA using heterogeneous catalysts. Herein, we report for the first time a long term test of a FA decomposition unit, using a novel, cost effective, robust and recyclable Ir containing catalyst, coupled with a hydrogen/air fuel cell (“indirect formic acid fuel cell”) to convert the chemical energy stored in FA to electricity with no gas purification requirement (Figure 1b, Figure 2).


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Biomass Formic Acid Storage

Renewable Energy

H2 Generation from HCO2H

e. g. solar, wind

Fuel cell Electricity Generation

electrons

Cycle

H2 CO2

CO2

HCO2H

Figure 2. A carbon neutral energy storage system based on formic acid.

Results and discussion In the course of preliminary experiments, it was found that application of iridium chloride as a sole catalyst precursor results in both a very limited FA decomposition rate and also poor selectivity (3.503 × 10-6 M s-1 and CO: 1.22 %, respectively, Scheme 1, Figure 3). IrCl3, Ligand, H2O HCO2H

CO2

H2

HCO2Na, 90-100 o C

Ligands:

N

N py

N

N N phen

bpy N

N

N

N

N PMDETA

TMEDA

HN

N

N N NH N

N NH N IndH

N

PAPH2

N

Scheme 1. Dehydrogenation of FA in the presence of IrCl3 and N-donor ligands.


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7 800

IndH

a)

2250

700 bpy

600

Pressure (psi)

PAPH2

400 300 PMDETA

1250 1000

0 2

4

6

8

10

12

14

16

750

105 ºC 100 ºC

500

250 90 ºC

py

0

112 ºC

1500

IrCl3 (no ligand)

TMEDA

100

Second run First run

1750

500

200

b)

2000

phen

Pressure (psi)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

18

20

Time (hour)

0 0

100 ºC 93 ºC 90 ºC

2

4

6

8

10

12

Time (hour)

Figure 3. Pressure versus time diagram for the decomposition of FA. a) First runs, catalyst precursors: IrCl3 and various N-donor ligands, T=100 ºC. b) IrCl3 in the presence of IndH, first and second run. [IrCl3]0=[Ligand]0=3.0 mM. Dashes divide the temperature segments, in the first run: 90 and 100 ºC, in the second run: 90, 93, 100, 105, 112 ºC, respectively.

In order to obtain a better catalyst performance, varied N-donor ligands were tested for enhancing the activity of IrCl3. The screening of the catalytic activity of IrCl3 for FA decomposition in the presence of N-donor ligands including the monodentate pyridine (py), the bidentate 2,2ʹ-bipyridine (bpy), 1,10-phenanthroline (phen) and N,N,N′,N′tetramethylethylenediamine

(TMEDA),

the

tridentate

N,N,N′,N′,N′′-

pentamethyldiethylenetriamine (PMDETA) and the multidentate 1,3-bis(2ʹ-pyridylimino)-isoindoline (IndH) and 1,4-di-(2ʹ-pyridyl)aminophthalazine (PAPH2) was carried out in a high pressure vessel. The list of applied N-donor ligands is shown in Scheme 1.

Decomposition of FA in the presence of IrCl3, mono- and bidentate N-donor ligands An aqueous solution of FA and HCO2Na was mixed with IrCl3·H2O and together with the corresponding N-donor ligand was charged into a Monel autoclave in a glove box purged with nitrogen. The reaction mixture was then heated to 90-112 °C. The pressure and the internal temperature of the reactor were monitored using Lab View 8.6 and reaction rates were calculated from the pressure versus time diagrams (Figure 3). After the decomposition reaction, the gaseous product was vented into a plastic bag and samples were taken for gas chromatographic and infrared spectroscopic analysis. Gas compositions and the calculated reaction rates are shown in Table 1.


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It was noticed that in the presence of some ligands (e.g. phen and also the tridentate PMDETA) a gradual deactivation of the catalyst occurred as indicated by the fast decline of the reaction rate. For these catalytic systems, the reaction rate was also calculated at a more advanced stage and the corresponding data are given in the footnotes of Table 1 and 2. Where relatively high activity and selectivity were observed in the first run, 15 g of 97% FA was added to the reaction mixture and the decomposition reaction (second run) was carried out again to study the robustness of the catalyst. The bidentate 2,2ʹ-bipyridine (bpy) in the presence of iridium chloride showed a relatively high activity (3.50 × 10-4 M s-1) already in the first run and no catalyst deactivation was observed based on the pressure versus time diagram. The good selectivity of the catalyst towards dehydrogenation of FA was reflected by the absence of carbon monoxide in the obtained gas mixture. After the addition of 15 g FA (97 %), the decomposition reaction was restarted and monitored again, showing an even higher activity in the second run (5.26 × 10-4 M s-1). However, the selectivity of the catalyst decreased resulting in a 0.1 % CO content in the gaseous product.

Table 1. Reaction rates for the FA decomposition reaction in the presence of IrCl3 and N-donor ligands and the chemical composition of gaseous products. Entry

Liganda

Gas composition (%)b H2

CO2

CO

106 × d[FA]/dt (M s-1)

1(F)

-

54.2

44.6

1.2

3.50

2 (F)

bpy

54.2

45.8

nd

350

3 (S)

bpy

52.6

47.3

0.1

526.

4 (F)

phen

43.7

56.3

nd

58.3c

5 (S)

phen

51.0

48.6

0.4

48.4d

6 (F)

TMEDA

54.3

45.5

0.2

3.70

7 (F)

py

54.7

44.9

0.4

1.39

a

Notes: (F) and (S) stand for first and second run, respectively, 0.075 mmol IrCl3·H2O, 0.075 mmol ligand, [FA]=3.6 M, [HCO2Na]=0.4 M, 25 mL aqueous solution, T=100 ºC. bDetermined by gas chromatography using TCD, nd: no CO was detected. c,dReaction rates measured after a more advanced reaction time: phen: c20.1 × 10-6 M s-1, d16.2 × 10-6 M s-1, respectively.

The other bidentate ligand, 1,10-phenanthroline (phen) gave one order of magnitude lower activity (5.83 × 10-5 M s-1) than the one observed using bipyridine. Very similar


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activity (4.84 × 10-5 M s-1) was observed in the second run after the addition of 15 g FA, indicating that the decrease in reaction rate (2.01 × 10-5 M s-1) in the last segment of the first run was reversible. A similar degree of deactivation was observed in the second run (reaction rate=1.62 × 10-5 M s-1). The application of phen resulted in similar selectivity in the first but lower selectivity in the second run (0.4 % CO) than that we observed using bpy. The relatively flexible bidentate ligand, tetramethylethylenediamine (TMEDA) in combination with IrCl3·H2O showed a very low rate of decomposition (3.70 × 10-6 M s1

) and poor selectivity (CO%=0.2). Because of the low reaction rate in the first run, the

activity of the catalyst was not studied further. The lowest decomposition rate (1.39 × 10-6 M s-1) and selectivity (CO%=0.4) was observed in the presence of IrCl3 and the monodentate pyridine (py) ligand. Although structural characterization of the catalysts formed in-situ was beyond the scope of this work, the FTIR spectra of the isolated solids were collected after reaction and showed broad peaks between 2100 and 1750 cm-1 indicating the presence of carbonyl complexes in the case of IrCl3/phen and IrCl3/TMEDA (Figure S1). Decomposition of FA using IrCl3 in the presence of tri/multidentate N-donor ligands In addition to mono and bidentate ligands, we attempted to study the catalytic activity of IrCl3 for FA decomposition in the presence of tridentate/multidentate N-donor ligands such

as

N,N,N′,N′,N′′-pentamethyldiethylenetriamine

(PMDETA),

1,4-di-(2'-

pyridyl)aminophthalazine (PAPH2) and 1,3-bis(2'-pyridyl-imino)-isoindoline (IndH). An aqueous solution of FA and HCO2Na mixed with IrCl3·H2O and the N-donor ligand was charged to a Monel autoclave and the reactor was closed in a glove box purged with nitrogen (Note: it will be indicated later that inert atmosphere is not necessary for the formation of the active catalyst). The reaction mixture was heated to 100 °C and the pressure and internal temperature of the reactor were monitored. Similar to the study of monodentate and bidentate ligands, reaction rates were calculated from the pressure versus time diagrams (Figure 3). The gas chromatographic analysis of the gaseous product showed the presence of hydrogen, carbon dioxide (≈ 1:1 ratio) and in some cases, carbon monoxide. Gas composition data and reaction rates are shown in Table 2. Although the decomposition of FA in the presence of PMDETA and IrCl3 took place at a reaction rate (4.86 × 10-5 M s-1) similar to that observed in the presence of 1,10phenanthroline, the loss of activity (reaction rate=5.70 × 10-6 M s-1) in the course of the


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reaction seemed to be irreversible. The selectivity of the catalyst was moderate in the first run, giving a CO content of 0.1 %. An improvement in the selectivity was observed using the PAPH2 ligand and IrCl3 (metal/ligand= 1:2) resulting in a non-detectable amount of CO and a reaction rate of 7.57 × 10-5 M s-1 that increased even further in the second run (18.9 × 10-5 M s-1). Even more promising results were obtained in the presence of a one to one equivalent mixture of IrCl3 and IndH as reflected by both the high activity (39.5 × 10-5 M s-1) and selectivity (Table 2). Despite a slightly lower activity measured in the second run (Figure 3b, 33.9 × 10-5 M s-1), the selectivity remained high. Applying a metal to ligand ratio of 1:2 further increased the activity in the first run (62.1 × 10-5 M s-1) but resulted in a significantly lower reaction rate in the second run (31.5 × 10-5 M s-1).

Table 2. Chemical composition of gaseous products and reaction rates for the decomposition of FA in the presence of IrCl3/tri-and multidentate ligands (PMDETA, PAPH2, IndH). Entry

Ligand

1 (F) 2 (F)

PMDETA 2 PAPH2

Gas composition (%)a H2% CO2% CO% 56.3 43.7 0.1 48.2 51.8 nd

105 × d[FA]/dt (M s-1) 4.86b 7.57

3 (S) 2 PAPH2 49.6 50.4 nd 18.9 5 (F) IndH 53.5 46.5 nd 39.5 6 (S) IndH 52.7 47.3 nd 33.9 7 (F) 2 IndH 53.1 46.9 nd 62.1 8 (S) 2 IndH 49.9 50.1 nd 31.5 Notes: (F) and (S) stand for first and second run, respectively. 0.075 mmol IrCl3·H2O, 0.075 (1 eq.) or 0.150 mmol (2 eq.) ligand, [FA]=3.6 M, [HCO2Na]=0.4 M, 25 mL aqueous solution, T=100 ºC. a Determined by gas chromatography using TCD, nd: no CO was detected, bThe reaction rate measured after a more advanced reaction time period PMDETA: 5.70 × 10-6 M s-1.

Based on the results of preliminary catalyst screening, the IrCl3:IndH=1:1 system was selected for further testing the recyclability, longevity and catalyst robustness under moderate pressure conditions in a continuous stirred reactor setup, as depicted in Figure S2. The reactor head assembly integrates a pressure gauge, an inlet for the neat FA through a needle valve and a pressure release valve that vents the evolving gas mixture continuously, once the pressure reached the selected cracking pressure (30 psi) of the pressure relief valve. The outlet of the pressure relief valve is connected to either a gas burette or the humidifier of the PEM fuel cell unit. An aqueous suspension of IrCl3·H2O, IndH, HCO2Na and FA was charged to the glass


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11 pressure reactor equipped with the modified head (Figure S2) and the reactor was closed under air. It should be emphasized that neither the precursors (including IrCl3·H2O and IndH) nor the in situ formed catalysts are air sensitive, adding a very significant advantage to this approach and rendering it more practical. The reaction mixture was gradually heated to 90 °C and the initially brown solution turned yellow. When the temperature reached 80 °C, gas evolution started and as the decomposition proceeded, the formation of fine black particles was observed. Once the internal pressure exceeded 30 psi, the pressure relief valve started to vent the gas mixture into a transfer line. The volume of the evolved gases was monitored by a gas volumetric apparatus giving an average flow of 0.5 L h-1. (Figure S2 and Figure 4a).

60 Volume of H2/CO2 mixture (mL)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-1

a) IrCl3-IndH (8.64 mL min ) 50 40 30 20

-1

b) IrCl3-IndH-CO (1.82 mL min )

10

-1

c) IrCl3 (0.43 mL min )

0 0

1

2

3

4

5

6

7

8

9

10

Time (min)

Figure 4. Volume of H2/CO2 mixtures versus time diagram for FA decomposition in the presence of various catalyst precursors. Note: These values were measured after the induction periods. [IrCl3]=[IndH]=9.987 mM, [FA]0=3.6 M, [HCO2Na]=0.40 M, 7.5 mL HCO2H/HCO2Na aqueous solution, T=90 ºC.

After reaction, the black precipitate (Ir-1) was separated and purified by washing with distilled water, repeatedly, and finally with millipore water. The microscopic images


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(SEM and TEM) and macroscopic appearance of the isolated Ir-1 are shown in Figure 5a-e. Elemental analysis of Ir-1 revealed that the isolated black material consisted of 23.28 % carbon, 4.83 % nitrogen, 19.79 % oxygen and 2.33 % hydrogen.

a)

b)

d)

e)

c)

f) Ir O C N

0.4

Al sample holder

1.2 2.8 2.0 Energy (keV)

3.6

Figure 5. The macroscopic appearance (a), the microscopic images (scale bar 10 µm (b), 1 µm (c), 1 µm (d), 50 nm (e)) and the EDX spectrum (f) of the isolated Ir-1 catalyst.

Energy-dispersive X-ray spectroscopic (EDX) measurements of Ir-1 confirmed the presence of Ir, C, O and N in the catalyst and also let us suggest the absence of chloride residues from the starting material (Figure 5f).Six peaks characteristic to iridium (Ir 4s (689.7 eV), Ir 4p3/2 (495.8 eV), Ir 4d3/2 (312.9 eV), Ir 4d5/2 (297.8 eV), Ir 4f (65.1 eV, doublet), Ir 5p3/2 (47.7 eV), Figure 6) were identified by X-ray photoelectron spectroscopy (XPS, Figure S3). Two well separated and symmetric spin-orbit components at 65.1 eV and 62.3 eV (∆E= 2.8 eV) led us to suggest the presence of iridium in its +2 or +3 oxidation state. Since iridium metal (Ir0) has asymmetric peak shape in the 4f region, the formation of Ir nanoparticles can be ruled out (Figure S3a). The peaks assigned to carbon and oxygen are not conclusive due to adventitious carbon adsorbed on the surface and also because of carbon tape used to hold the powder sample (Figure S3b and c). The N 1s peak was observed at 398.6 eV, which can be assigned to organic nitrogen (Figure S3d). A very low intensity peak (197.6 eV), almost indistinguishable from the background signal, can be attributed to Cl 2p, indicating negligible amount of chloride residue from IrCl3.


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13

2

6 1

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3

4

7 8

5

9

1000

800

600

400

200

0

Binding Energy (eV)

Figure 6. XPS survey spectrum of Ir-1 catalyst. (1) Ir 4s, (2) O 1s, (3) Ir 4p3/2, (4) N1s, (5) Ir 4d3/2, (6) Ir 4d5/2, (7) C1s, (8) doublet, Ir 4f5/2, Ir 4f7/2, (9) Ir 5p3/2. The FTIR spectrum of Ir-1 (Figure 7b) showed an intensive broad peak at 2094-1956 cm-1 suggesting the presence of carbonyl functional groups. The powder x-ray diffraction measurements of the catalyst (Ir-1) does not show significant degree of diffraction (Figure S4) due to its non-crystalline nature. NMR measurements did not provide any valuable information because of the very low solubility of Ir-1 in organic solvents. The diffuse reflectance UV-Vis spectrum was recorded on the mixture of Ir-1 catalyst and MgO. However, this analysis did not provide any assignable peak (Figure S5). Thermogravimetric analysis revealed that the main thermal degradation process takes place on Ir-1 in the temperature range of 200-300 ºC, in which the sample lost 42.7 % of its original weight (Figure S6). The Ir content of the sample can be estimated as 49.7 % based on the residual weight of the combusted sample. In order to gain more insight on how the ratio of catalyst precursors impacts the catalytic activity, the decomposition reactions were repeated in the absence of IndH and in the presence of IndH and external CO (60 psi). Since it was noticed earlier that the in situ formed catalyst contains carbonyl ligands, it was expected that the addition of


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external CO would accelerate the formation of the catalytically active complexes, shortening the induction period. Contrary to our assumption, in the presence of CO, the gas evolution rate (1.82 mL min1

) was more than fourfold lower than in the absence of CO (8.64 mL min-1), indicating

the formation of a different catalytic system (Figure 4b). An even lower gas evolution rate was observed in the absence of both IndH and CO using only IrCl3 (0.43 mL min-1, Figure 4c). The FTIR study of the isolated black particles confirmed the presence of coordinated carbonyl ligands even in the absence of external carbon monoxide. (Figure 7). It was found that the higher the relative intensity of the peak at 2018 cm-1, the lower the catalytic activity. It can be suggested that the presence of IndH suppresses the formation of the inactive species favoring the generation of the active one at 2057 cm-1. a)

IrCl3

b)

IrCl3-IndH

c)

IrCl3-IndH-CO

3500

3250

3000

carbonyl

2750

2000

1750

1500

1250

-1

Wavenumber (cm )

Figure 7. FTIR spectra of the solids obtained from the reaction between IrCl3 (IndH and CO) in aqueous formic acid and sodium formate.

The formation of carbonyl complexes even in the absence of CO may be explained by the ability of FA to act as the source of CO in the carbonylation reactions of transition metal complexes. Similar routes were reported for the formation of several Ru-carbonyl complexes

such

[Ru(HCO2)2(CO)2(PPh3)2],48,62

as

[Ru2(HCO2)2(CO)4(PPh3)2],48,62

and

even

for

larger

cluster

[Ru(CO)3(PPh3)2],48 compounds

like

[Ru4(CO)12H4]47 and [Ru12C30H14Na2O50·6(C18H15OP)·2(C7H8)·4(H2O)]48 in previous studies by us and other research groups.


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Continuous FA decomposition under moderate pressure coupled to a hydrogen-air PEM fuel cell (indirect formic acid PEM fuel cell) Guided by the findings of catalyst screening, a continuous FA decomposition reactor was built and coupled with a hydrogen/air PEM fuel cell to demonstrate that FA can be continuously converted to electricity employing an indirect formic acid fuel cell (IFAFC). In the former experimental setup, FA was occasionally added to the reactor with a syringe resulting in fluctuating FA concentration and consequently unstable gas flow in longer duration experiments. In order to suppress the fluctuation of the gas flow due to the irregular FA concentration, a dual syringe pump (Figure S7) was installed to feed neat FA to the reactor continuously. The rate of the FA addition to the reactor was adjusted to the rate of gas evolution in order to avoid the accumulation or the depletion of FA in the reaction mixture. In a scaled up experiment, an aqueous suspension of IrCl3·H2O, IndH, HCO2Na and FA was charged in the modified pressure tube reactor, closed under air and heated to 100 °C (Figure S8). The rate of gas evolution was determined using a gas volumetric apparatus (≈6.6 L h-1) and checked regularly in the course of the experiment. It was found that the catalyst maintained its activity, for the duration of this study (20 days) as shown in Figure S9. Moreover, similar activity was obtained one year after the original synthesis, when the catalyst was kept in the reaction mixture under H2/CO2 pressure. Additional three years of storage under ambient conditions induced only a loss of 25% of the original activity. The gas chromatographic analysis of the produced gas mixture using a thermal conductivity detector revealed a 1:1 ratio of H2/CO2 and the absence of carbon monoxide (detection limit of the TCD was 0.099 v/v %). FTIR analysis of the gas mixture further confirmed the lack of CO in the product gas mixture (Figure S10). The mixture of H2 and CO2 obtained by decomposition of FA using IrCl3/IndH catalyst was fed into a hydrogen-air PEM fuel cell to demonstrate that the chemical energy stored in FA can be converted to electricity. A comprehensive study on the performance of the hydrogen-air fuel cell coupled to FA decomposition reactor is beyond the scope of this paper. Nevertheless, satisfying results are shown below to demonstrate that the described system using a newly developed Ir containing catalyst is able to supply a constant flow of a H2/CO2 mixture with no CO impurity, making it an appropriate


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16 anodic feed for a hydrogen-air fuel cell to produce electricity continuously. Experimental details for the preparation of the proton exchange membrane electrodes and the fuel cell assembly are provided in the experimental section and in the supporting material (Figure S11-S13). The stable performance of the newly assembled fuel cell was demonstrated by monitoring the cell voltage as a function of time at constant current (I= 1.0 A) using ultra high purity (UHP) hydrogen as a fuel from a cylinder. These measurements were considered as benchmarks to compare the performance of the fuel cell after extensive use under various conditions as shown below. As it can be seen in Figure 8 (inset), the cell voltage was stable (0.85 V at I=1.0 A) for the time of the measurement using either O2 or air as cathode feed gas.

1.0

-1

100 mL min H2/CO2(from FA decomposition), -1

240 mL min Air

0.8

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.6

1.0

0.9 H2 (UHP, cylinder)/O2 H2 (UHP, cylinder)/Air

0.8

0.4 0.7

0.2

0.0 0.0

0.6 0.0

0.2

0.2

0.4

0.4

0.6

0.6

0.8

1.0

0.8

1.0

Time (hour)

Figure 8. Fuel cell voltage versus time at constant current (I=1.0 A). Treactor=100 ºC, anode feed gas: 100 mL min-1 H2/CO2, cathode feed gas: 240 mL min-1 air, Thumidifier=85 °C, Tfuel cell=65 °C. [IrCl3]0=[IndH]0=12.72 mM. Inset: cell voltage as a function of time. Conditions are similar to those indicated in Figure 8, except H2 flow=60 mL min-1, cathode feed gas: 240 mL min-1. In order to design a practical energy storage system based on FA as an energy carrier, the ultimate oxidant (cathode feed gas) is preferably air, which is more cost effective than oxygen. The integrated FA decomposition-fuel cell system was tested using air as cathode feed gas and no difference in performance could be observed under the given experimental conditions compared with H2/O2 or H2/air fuel cell (Figure 8).


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17 The current versus time diagram (Figure 9) shows that the fuel cell maintained good stability under the applied experimental conditions and that the cell responded to a higher flow rate (100 → 110 mL min-1) with a higher current value (5.95 → 6.95 A). The durability of the coupled system was demonstrated in a separate, longer duration experiment, where the fuel cell maintained its voltage (0.85 V) at a current value of 1.0 A, for the entire 14 hours of the experiment (Figure 9, inset). During this time period 75. 6 Litres of H2/CO2 mixture was supplied to the cell. -1

110 mL min H2/CO2 -1

100 mL min H2/CO2

7 6

4 3 2

Voltage (V)

5

Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.0

-1

90 mL min H2/CO2

0.9 0.8 0.7

2

0.6

1 0 0.0

4

6

8

10

12

14

Time (hour)

0.5

0.2

0.4

0.6

0.8

1.0

Time (hour)

Figure 9. Fuel cell current as a function of time. Anode feed gas: 100 and 110 mL min-1 H2/CO2, cathode feed gas: 240 mL min-1 air, Thumidifier=85 °C, Tfuel

cell=65

°C. Inset:

extended time cell voltage versus time diagram. Conditions are similar to those indicated in Figure 9, except H2/CO2 flow (90 mL min-1). [IrCl3]0=[IndH]0=12.72 mM. It is known from the literature that CO2 reduction on Pt (100) and Pt(110) surfaces results in CO as a reduction product.74,75 Smolinka et al.76 carried out a comparative study on a polymer electrolyte fuel cell (PEFC) using pure H2 (a), synthetic reformate (H2/CO2 (25 %)), (b) and CO contaminated anode feed gases. Only a small loss in voltage (30 mV at 0.5 A cm-2) was observed when switching from pure hydrogen to H2/CO2 gas mixture indicating that hydrogen oxidation is affected only by a “dilution effect” due to the lower partial pressure of H2. The authors proposed that CO2 reduction on Pt under the given experimental conditions becomes kinetically limited at adsorbed CO coverages of 0.6 where the hydrogen oxidation is still facile. On the other hand, severe performance losses were detected, when the feed gas was contaminated with 50 ppm CO indicating that the steady state CO coverage is high enough to hinder the


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hydrogen oxidation reaction. Also, there are some established methods to mitigate CO poisoning of PEM fuel cell membranes. For example poly-benzimidazole membranes allow for higher operating temperatures (100-120 ºC) that increase CO tolerance of the fuel cell Pt catalyst77 while the deleterious effect of CO can also be suppressed by injecting low amount of O2 into the CO contaminated H2 feed gas in PEM fuel cells at lower operating temperatures (80 ºC).78,79 Comparing the fuel cell performances using pure hydrogen as an anode feed gas with those obtained using an H2-CO2 mixture from the FA decomposition unit, it could be assumed that purification of the gas mixture can be avoided and this mixture is appropriate for direct use in a PEM fuel cell. It should also be stressed that in a practical energy/hydrogen storage system, carbon dioxide that is formed in FA decarboxylation and passed through the fuel cell, has to be recycled, rendering the overall storage “carbon neutral” as depicted in Figure 2. In order to provide further support for the efficient use of H2-CO2 gas mixture in the PEM fuel cell, the product gas from FA decomposition unit was subjected to CO2 removal by bubbling the gas mixture into NaOH solution and the resulting effluent gas was fed into the anodic side of the PEM fuel cell (Figure S14). No significant difference was observed in the performance of the fuel cell using the purified or the non-purified gas mixtures (Figure S15). The maximum power density that we measured in the fuel cell experiments (150-175 mW/cm2 at a potential of 0.5 V, Figure S16) is significantly higher than the value (47 mW/cm2 at a potential of 0.347 V) reported previously for a similar system integrating FA decomposition ([RuCl2(PPh3)4] catalyst) and electricity generation in a hydrogen-air fuel cell.63,80 An additional advantage of the present integrated system is that the generated mixture of H2 and CO2 was used directly in a fuel cell without the requirement of any further purification, contrasting with the study reported earlier in the literature,63,80 where volatile amines present in the system had to be removed by passing the generated gas through charcoal prior to utilization as a fuel. One may notice the analogy of the indirect formic acid fuel cell concept with Reformed Methanol Fuel Cells (RMFC, also called Indirect Methanol Fuel Cells (IMFC)) which are subcategories of proton-exchange fuel cells. In RMFC for transportation applications, an on-board reformer81 converts methanol into hydrogen and carbon dioxide. The gas mixture is subsequently used in a hydrogen/air fuel cell to generate


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electricity. The two-step methanol utilization pathway offers an advantage by overcoming the most typical issues with direct methanol fuel cells, e.g. methanol crossover, and high catalyst loadings. The question arises whether IMFCs or indirect formic acid fuel cells (IFAFCs) are more advantageous for chemical energy storage? In order to answer this question both IMFCs and IFAFCs should be tested as part of an integrated energy storage system. At the present stage of the development, however, limited comparison can be made. On board methanol fuel processors are relatively complex systems involving the autothermal reactor (ATR), high-temperature and low-temperature water–gas–shift (HTS, LTS) reactors. The final stage before reformate is sent to the fuel cell is a preferential oxidation (Prox) unit, which is responsible for the oxidation of CO to CO2. A tail gas combustor unit (TGC) is also necessary to burn the unconverted hydrogen after the fuel cell (this unit is also required for FA decomposition/H2-air fuel cell systems).81 The complexity of these systems brings up some difficulties to build a space efficient IMFC. However, space issues are less important for (larger scale) energy storage applications. Indirect formic acid fuel cells offer some advantages over indirect methanol fuel cell systems. IFAFCs operate at significantly lower temperature 90-110 ºC while methanol reforming requires temperatures as high as 250-300 ºC, increasing the cost of heat management, insulation, etc. Another important parameter for IMFCs is the start-up time viz. the time necessary for reaching the target CO level (100 ppm) in the H2 fuel.81 Since the reformed gas containing CO levels higher than 100 ppm cannot be used in the PEM fuel cell operating at low temperature (< 80 ºC), a careful CO monitoring system has to be installed. Although enormous progress has been achieved in the start-up time of IMFCs, it is still in the minute range, while IFAFC using our catalyst provides low CO levels even during the heating up period when the gas flows are still low. On the other hand energy storage applications based on the suggested IFAFC system may face other problems related to the higher freezing temperature of FA (8.4 ºC) compared with methanol (-97 ºC).

Conclusions After screening various cyclic and non-cyclic, flexible and relatively rigid backbone Ndonor ligands, IrCl3·H2O in combination with 1,3-bis(2’-pyridyl-imino)-isoindoline (IndH) was found to be the most advantageous catalyst precursor for a practical FA


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decomposition reaction to H2 and CO2. It was shown that a heterogeneous catalyst formed from IrCl3·H2O and IndH precursors in the presence of FA and sodium formate in an aqueous solution has high activity, selectivity and stability for the dehydrogenative cleavage of FA. The catalytic system maintained its activity for the duration of the study, 20 days, and retained a similar activity even a year after its original preparation. The very impressive activity, stability, robustness and ease of recyclability of the catalyst enabled the construction of an integrated FA decomposition and hydrogen/air PEM fuel cell system to demonstrate the practicality of this approach by producing electricity continuously from FA. The catalyst is easy to reuse by adding simply neat FA to the reactor without the requirement of its separation from the reaction mixture. The fuel cell was tested using the H2-CO2 mixture directly from the FA decomposition unit without further purification and the fuel cell delivered very similar performance to those experiments where pure H2 from a cylinder was used as a feed gas. The fact that the feed gas does not need purification would significantly reduce the operating cost of a practical energy storage system using FA as a hydrogen/energy carrier. It is also worth noting that the preparation of the active and stable catalyst from IrCl3·H2O and IndH precursors does not require special (inert) reaction conditions and it can be performed under air using commercial HCO2Na and FA. The described integrated system embodies the first durable and efficient indirect formic acid PEM fuel cell, which is able to convert the stored chemical energy of FA into electricity for extended periods of time without the requirement of gas purification and inert conditions. Experimental section The description of chemicals and general methods can be found in the Supporting Information.

General procedure for the decomposition of FA in the presence of IrCl3 and N-donor ligands in a high pressure reactor

IrCl3·H2O (27.7 mg (0.075 mmol)) and the ligand (0.075 mmol) were suspended in 25 mL of a 9:1 mixture of 4.0 M aqueous solution of FA and 4.0 M solution of HCO2Na and charged into a Monel autoclave (125 mL) and the reactor was closed in a glove box purged with nitrogen. The reaction mixture was heated gradually to the required temperature, the pressure and internal temperature of the reactor were monitored using Lab View 8.6 software. After the decomposition reaction, samples were drawn from the


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gaseous products and subjected to GC analysis.

Decomposition of FA in the presence of IrCl3 and IndH in a pressure tube reactor IrCl3·H2O (27.7 mg (0.075 mmol)) and IndH (22.4 mg, 0.075 mmol) were suspended in 7.5 mL of a 9:1 mixture of 4.0 M aqueous solution of FA and 4.0 M solution of HCO2Na and charged to a modified pressure tube reactor with a volume of 38 mL (Figure S2) followed by heating the reaction mixture to 100 ºC. The pressure relief valve of the reactor was set to 30 psi. The outlet of the reactor was connected to a gas volumetric apparatus in order to determine the flow rate of the produced H2/CO2 mixture.

Experimental procedure for the fabrication of the fuel cell membrane electrode assembly Carbon paper (Toray) was sectioned into 50×50 mm size pieces for coating with the catalysts. The catalyst powder (Pt, 20 %), Nafion suspension (20%) and Milli-Q water (60%) were mixed in a glass vial, and subjected to sonication for about 15 minutes to make a fine suspension. The suspension was brushed on the carbon paper and dried under a stream of air at ambient temperature. Brushing and drying were repeated to get a catalyst loading of 8 mg/cm2. The Pt coated carbon paper electrodes were used both as the anode and the cathode. A Nafion membrane (E-Tech) was placed between the obtained coated carbon papers and pressed at 140 °C for 45 minutes, applying a pressure of 130 psi in a PHI press (Figure S11). The membrane electrode was placed between graphite separators and conditioned by flowing water through both the anode and the cathode side at room temperature. The PEM fuel cell is depicted in Figure S12 and S13 in the SI. Combined FA decomposition/fuel cell experiment The FA decomposition reaction was scaled up in order to provide a sufficient amount of feed gas for the H2/air fuel cell. 0.3902 g (1.056 mmol) IrCl3·H2O and 0.3154 g (1.056 mmol) IndH were added to the mixture of 83 mL aqueous FA/HCO2Na solutions (9:1 mixture of 4M FA and 4M HCO2Na). The suspension was transferred into a modified pressure tube reactor with a volume of 185 mL (Figure S2) and heated to 100 °C. The original brown reaction mixture turned into a yellow solution and gas evolution ensued


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when the temperature reached 90 °C. The formation of black particles was observed after 20 minutes. A flow rate of 146 mL min-1 (H2/CO2) was observed at the beginning. After this, the activity started to subside and stabilized at 110 mL min-1. The system maintained similar activity for the duration of this study (20 days). The vent of the pressure relief valve on the FA decomposition unit was connected to the H2/air PEM fuel cell through a humidifier in order to prevent the loss of proton conductivity of the Nafion membrane due to dry-out. Figure S8 illustrates the experimental setup. A Fuel Cell Test System 890B (Scribner Associates Inc.) was used to monitor the performance of the fuel cell.

AUTHOR INFORMATION Corresponding Authors *[email protected], [email protected], [email protected] ACKNOWLEDGMENT Support of our work by the Loker Hydrocarbon Research Institute and the US Department of Energy is gratefully acknowledged. XPS data and SEM and TEM images presented in this article were acquired at the Center for Electron Microscopy and Microanalysis at the University of Southern California.


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Iridium-Catalyzed Continuous Hydrogen ... - ACS Publications

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